The present invention relates to a method and apparatus for attaching a printing plate onto a plate cylinder of a printing press. Specifically, the invention relates to a method and apparatus for attaching a flexible printing plate prepared by digital plate making.
In a printing press for lithographic printing, it is a common practice to wind and fix a printing plate around a plate cylinder and carry out printing with the plate kept in such a condition.
However, a lithographic printing plate, which is inexpensive and easy to handle and at least the rear surface of which is made of a material other than metal (hereinafter referred to as a flexible printing plate), has a drawback in dimensional stability. For example, the flexible printing plate has a problem in that distortion may occur by handling of the printing plate at the plate attachment or by a frictional force generated between the printing plate and a blanket cylinder during printing and as a result, the print dimension and the accuracy in the printed position relative to printing paper are degraded.
Accordingly, the use of such a flexible printing plate has been limited to a simple printing for a relatively small number of prints wherein a high registration accuracy in the resulting print is not so severely required, and such flexible printing plates have not been used for multi-color high-quality printing and printing using a large printing press.
The above-mentioned distortion of a flexible printing plate caused by the handling at the plate attachment is described in detail below. In FIG. 16, a typical plate-attaching unit 60 is shown. FIG. 16 shows a view of the plate-attaching unit when it is viewed from the direction of the arrow X in FIG. 15. FIG. 15 is a cross-sectional view taken along the line Yxe2x80x94Y shown in FIG. 16.
As is shown in FIG. 15, a plate cylinder 61 of the printing press has such a shape as being obtained by cutting off a part of a cylinder along the axial direction thereof and providing a groove 65 in a substantially flat plane (hereinafter referred to as a flat portion 61a), which is the cut edge of cylinder. In the groove 65 are provided a plate front end clamping mechanism 62 that clamps and holds the front end or leading edge of a printing plate 70 and a plate rear end clamping mechanism 63 that clamps and holds the rear end or trailing edge of the printing plate 70. Each of the clamping mechanisms 62 and 63 is provided with a lower tooth 62b or 63b the upper surface of which constitutes substantially the same level as the flat portion 61a, and an upper tooth 62a or 63a which is rotatably supported by the bottom tooth and capable of clamping and holding the front end of printing plate between the upper surface of the bottom tooth and itself. Further, a position-adjusting mechanism (not shown), which can control the positions of the plate front end clamping mechanism 62 and the plate rear end clamping mechanism 63 in the forward-backward, left-right or oblique direction is provided on the bottom plane of the groove 65.
There is formed a ridge portion 61c between the flat portion 61a and a curved portion 61b of the plate cylinder 61. Though not shown, there is known a plate cylinder in which, for example, the upper surface of the lower tooth in the clamping mechanism, instead of the flat portion, forms a ridge portion together with the curved portion therebetween.
Usually, the portion where the flat portion 61a and the curved portion 61b are bounded is rounded to form a curved surface with a radius of 5 to 30 mm. Such a rounded portion is also referred to as the ridge portion in the specification.
As shown in FIG. 16, on the upper surface of the lower tooth 62b of the plate front end clamping mechanism, protruding registration pins 64 are provided at two positions with a spacing in the axial direction of the plate cylinder 61. At the positions of the upper tooth 62a of the plate front end clamping mechanism corresponding to those of the registration pins 64, notches 66 are formed respectively. Further, in a plate front end 70a of the printing plate 70 shown by a two-dot chained line in the figure, notches 71 at the positions corresponding to those of registration pins 64.
In the case where a flexible printing plate 70 is attached onto the plate cylinder 61 using, for example, the plate-attaching unit 60 described above, an operator pushes the plate front end 70a of the printing plate into a gap between the upper tooth 62a and the lower tooth 62b of the plate front end clamping mechanism so that the plate front end comes into contact with the two registration pins 64 while the operator holds the plate rear end 70b in his hand. At that time, it accidentally happens that the plate front end 70a is deformed and fixed in such a deformed state. Specifically, since distance between the two registration pins 64 aligned on plate cylinder 61 is considerably large in the plate cylinder capable of attaching a printing plate of a half-kiku size (approximately 670 mm in width and 560 mm in length) or larger, when the operator pushes a flexible printing plate 70 even slightly in excess at the plate attachment, deformation of the printing plate 70 occurs in such a manner that the plate front end 70a protrudes from the spacing between the two registration pins 64 in the pushed direction. However, when the printing plate 70 is pushed insufficiently with care of preventing such deformation, accurate positioning of the printing plate is impossible.
Moreover, as shown in FIG. 15, in the vicinity of the ridge portion 61c formed between the flat portion 61a and the curved portion 61b of the plate cylinder 61, the flexible printing plate 70 is difficult to adjust itself to the outer periphery of plate cylinder 61, and tends to float from the outer periphery of the plate cylinder.
To prevent such floating of the printing plate 70, the operator wraps and attaches the printing plate 70 by rotating the plate cylinder while strongly pulling the plate rear end of the printing plate whose front end has been clamped and held by the plate front end clamping mechanism 62. However, during such operation, the pulled portion in the plate rear end of the flexible printing plate 70 is elongated.
After winding and attaching the printing plate by pulling the plate rear end thereof as described above, the plate rear end of the printing plate is pushed into a gap between the upper and lower teeth of the plate rear end clamping mechanism and fixed by rotating the upper tooth. Then, by moving the plate rear end clamping mechanism in such a direction as to impart tension to the printing plate using a tension-applying mechanism (not shown), the printing plate is brought into close contact with the periphery of plate cylinder, whereby the plate attachment is completed.
In a conventional plate-attaching device, such a tension-applying mechanism for bringing a printing plate into close contact with the peripheral surface of plate cylinder is designed in conformity with a printing plate having a metal substrate such as aluminum. Accordingly, in the case of a flexible printing plate, the plate is elongated to an extent more than necessary.
On the other hand, when the tension-imparting mechanism is designed so as to apply a low tension adapted for flexible printing plates, the tension is not enough and the printing plate fails to get in close contact with the plate cylinder. Even when the printing plate is favorably attached without floating from the plate cylinder, under such a weak tension the printing plate is moved by the frictional force between the blanket cylinder and itself during printing.
As means for resolving the above-described problem of moving printing plate (hereinafter referred to as xe2x80x9cplate slippagexe2x80x9d), there have been proposed a method of immobilizing the plate on the plate cylinder using a spray paste (Japanese Patent Laid-Open No. 11258/1981), and a method of immobilizing a plate using an adhesive material (e.g., Japanese Patent Publication No. 425/1995 and Japanese Patent Laid-Open No. 104853/1986). But, these methods are not effective from practical viewpoint, since fine position adjustment needed subsequently becomes quite difficult once the plate is bonded to the plate cylinder.
A plate mounting method is proposed in Japanese Patent Laid-Open No. 20130/1999 wherein the positional and dimensional accuracies under high-speed printing are improved by using a sheet material having an initial modulus of elasticity not higher than 300 kgf/mm2 and wherein free slide of the printing plate due to the reduction of the frictional resistance between the plate cylinder and the printing plate is secured by further providing a concavo-convex (matte) surface through a chemical or physical surface treatment or bonding fine particles on the surface of sheet material, whereby smooth registration adjustment can be practiced. However, it has been found that, though this technique enables position registration as well as a fixed clamping, it cannot prevent plate slippage satisfactorily.
On the other hand, a sheet material having a centerline roughness Ra of 2 xcexcm or larger on at least one surface thereof (Japanese Patent Laid-Open No. 193828/1998) and a packing sheet having a concavo-convex surface provided with a distribution of specified minute protrusions on the surface of a sheet-formed base material (Japanese Patent Laid-Open No. 59012/1999) are proposed. These packing sheets can prevent plate slippage through biting into the back surface of a lithographic printing plate, and further enable delicate position adjustment due to the effect of the surface concavities and convexities. However, even in such packing sheets, the effect of preventing plate elongation and displacement is still insufficient, and the development of more improved method has been expected.
In these years in the field of lithographic printing, many proposals have been made on the system that directly outputs digital image information on a printing plate, owing to the recent improvement of digital imaging technology and the request of raising process efficiency. Such technique is called CTP (Computer-To-Plate) or DDPP (Digital Direct Printing Plate).
The above-described technique has the advantages of achieving good positional accuracy of image with respect to a printing plate and a good registration accuracy in multi-color printing, compared to a conventional plate-making method (in which contact exposure is performed by superimposing a lith film on a printing plate precursor).
As a matter of course, digital plate-making systems using a flexible plate precursor have also been proposed, and owing to their low prices, there exists needs for multi-color printing using such systems. However, since the flexible printing plate have the above-described problems, they cannot make full use of the advantage of good positional accuracy of image resulting from the digital plate-making system. Thus, in multi-color printing, the flexible printing plate has not achieved the same registration accuracy as that of a printing plate having a metal support.
An object of the invention is to provide a plate-attaching method capable of readily attaching a flexible printing plate prepared digitally without plate distortion, capable of putting into practice with a slight improvement of an already installed printing press, providing the printing plate with a good attached position accuracy and capable of preventing the plate slippage and staining in the non-image area during printing.
Other objects of the invention will become apparent from the following description.
The above-described objects of the invention are attained by the following items:
1. A method of attaching a flexible printing plate on a plate cylinder comprising:
providing a flexible printing plate having a concavo-convex structure on the rear surface thereof and three or more notches at the plate front edge thereof;
providing an apparatus of attaching a flexible printing plate comprising:
a plate front end positioning mechanism comprising:
a plate front end clamping mechanism having upper teeth and lower teeth for fixing the plate front end of said printing plate to said plate cylinder; and at least three registration pins respectively provided in said plate front end clamping mechanism in such a manner in that they are respectively opposed to said notches of said printing plate;
a printing plate pressing mechanism for pressing said printing plate toward said plate cylinder;
a plate rear end fixing mechanism for fixing the plate rear end of said printing plate to said plate cylinder; and
a packing sheet having a concavo-convex structure at least on one surface thereof and being wound and fixed around said plate cylinder in such a manner that the surface having the concavo-convex structure is faced said printing plate;
positioning and fixing said plate front end to said plate cylinder by inserting said plate front end of said printing plate between said upper and lower teeth of said plate front end clamping mechanism to contact inner peripheral surface of said notches of said plate front end with said at least three registration pins;
winding said printing plate on said plate cylinder by rotating said plate cylinder with said plate rear end kept free, while said printing plate pressing mechanism presses said printing plate toward the entire outer peripheral surface of said plate cylinder including a ridge portion thereof using a pressure member; and
pressing and fixing said plate rear end of said printing plate in the radial direction of said plate cylinder using said plate rear end fixing mechanism after the completion of said winding;
whereby elongation of said flexible printing plate in its maximum printing length in the rotational direction of said plate cylinder during said printing plate attaching operation is controlled not more than 100 xcexcm.
2. A method of attaching a flexible printing plate on a plate cylinder comprising:
providing a flexible printing plate having a concavo-convex structure on the rear surface thereof and at least three fixing holes at the plate front edge thereof;
providing an apparatus of attaching a flexible printing plate comprising:
a plate front end positioning mechanism comprising:
a plate front end clamping mechanism having upper teeth and lower teeth for fixing the plate front end of said printing plate to said plate cylinder; registration pins provided in said plate front end clamping mechanism; and
a pin contact member, which has higher rigidity than said printing plate, is attached to said registration pins and has at least three fixing pins for fixing said printing plate;
a printing plate pressing mechanism for pressing said printing plate toward said plate cylinder;
a plate rear end fixing mechanism for fixing the plate rear end of said printing plate to said plate cylinder; and
a packing sheet having a concavo-convex structure at least on one surface thereof and being wound and fixed around said plate cylinder in such a manner that the surface having the concavo-convex structure is faced said printing plate;
positioning and fixing said plate front end to said plate cylinder by inserting said respective fixing pins of said pin contact member between said upper and lower teeth of said plate front end clamping mechanism into said respective fixing holes of said printing plate;
winding said printing plate on said plate cylinder by rotating said plate cylinder with said plate rear end kept free, while said printing plate pressing mechanism presses said printing plate toward the entire outer peripheral surface of said plate cylinder including a ridge portion thereof using a pressure member; and
pressing and fixing said plate rear end of said printing plate in the radial direction of said plate cylinder using said plate rear end fixing mechanism after the completion of said winding;
whereby elongation of said flexible printing plate in its maximum printing length in the rotational direction of said plate cylinder during said printing plate attaching operation is controlled not more than 100 xcexcm.
3. A method of attaching a flexible printing plate on a plate cylinder comprising:
providing a flexible printing plate having a concavo-convex structure on the rear surface thereof and at least three fixing holes at the plate front edge thereof;
providing an apparatus of attaching a flexible printing plate comprising:
a plate front end positioning mechanism comprising:
a plate front end clamping mechanism having upper teeth and lower teeth for fixing the plate front end of said printing plate to said plate cylinder; and registration pins provided in said plate front end clamping mechanism;
a printing plate pressing mechanism for pressing said printing plate toward said plate cylinder;
a plate rear end fixing mechanism for fixing the plate rear end of said printing plate to said plate cylinder;
a packing sheet having a concavo-convex structure at least on one surface thereof and being wound and fixed around said plate cylinder in such a manner that the surface having the concavo-convex structure is faced said printing plate; and
a pin contact member which has higher rigidity than said printing plate and has at least three fixing pins for fixing said printing plate;
inserting said fixing pins of said pin contact member into said fixing holes of said printing plate, respectively, to fix;
positioning and fixing said printing plate to said plate cylinder by fixing a leading edge of said pin contact member to said registration pins between said upper and lower teeth of said plate front end clamping mechanism;
winding said printing plate on said plate cylinder by rotating said plate cylinder with said plate rear end kept free, while said printing plate pressing mechanism presses said printing plate toward the entire outer peripheral surface of said plate cylinder including a ridge portion thereof using a pressure member; and
pressing and fixing said plate rear end of said printing plate in the radial direction of said plate cylinder using said plate rear end fixing mechanism after the completion of said winding;
whereby elongation of said flexible printing plate in its maximum printing length in the rotational direction of said plate cylinder during said printing plate attaching operation is controlled not more than 100 xcexcm.
4. A method of attaching a flexible printing plate set forth in any one of items 1 to 3, wherein the concavo-convex structure of said packing sheet has Ra (Ra: centerline average roughness) of less than 2 xcexcm.
5. A method of attaching a flexible printing plate set forth in any one of items 1 to 4, wherein the concavo-convex structure of said packing sheet has Ra/Rz (Rz: ten-point average roughness) of less than 0.17.
6. A method of attaching a flexible printing plate set forth in any one of items 1 to 3, wherein a concavo-convex structure of the rear surface of said printing plate has Ra value of less than 1.5 xcexcm.